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This is not the only way to optimize a divine caster, nor can it be considered the best. It does, however, produce a massively powerful character capable of fighting on the front lines at least as well as a typical martial character, and extremely deadly to undead foes.

With 6 levels of Cleric, you meet the requirements for Radiant Servant of Pelor. This Prestige class continues to improve all the Cleric's abilities and gives further bonuses and power boosts to those that weren't already powered up, as well as martial weapon proficiencies. You thus reach 17th level with all the same abilities as a 17th level Cleric, plus the substantial bonuses from the prestige class. From here, feel free to take 3 levels as a Hierophant, or a Shadowstriker, or some combination of martial classes, or even round out your Cleric levels. Preparing a Reach spell lets you use Persistent spell for it.

It's Clericzilla. It's already munchkin-sized. But if you really want your GM to throw the book at you add on the Saint template as long as LA buy off is allowed. Then only question is if you wamt to take your last levels in Hierophant for more options in spellcasting, or in Shadowstriker for some Paladin-esque abilties, or as a Ranger and Fighter for feats and favored enemy (undead), or just top out your Cleric levels and be more of a caster. This is assuming you're not going into epic levels, of course.

if you take Hierophant levels you can get the Divine reach ability which gives your touch spells a set range for Persistent spell with no increase in spell level.

I would rather improve the casting ability with some metamagic rather than take power attack. At this level optimized fighters are way beyong what you can get from a cleric anyway. Also, there are prestige classes that should be considered instead of the 6th and 7th levels of cleric.

Pelor doesn't have access to the Planning Domain. This isn't to say this build is impossible, you just can't get some of the feats listed because you have to get Extend Spell yourself to have access to Persistent Spell.